1. Field of the Invention
This invention relates to a superconducting system for creating a superconducting perpetual current loop for use in a nuclear magnetic resonance medical diagnosis apparatus (NMR-CT) or a power storage magnet that converts power into magnetic energy and stores the same.
2. Discussion of Background
FIG. 6 is a schematic diagram illustrating a conventional superconducting system that creates a perpetual current loop. The conventional superconducting system is constituted by a power source 2 for a superconductive coil 1 (hereinafter simply referred to as a DC power source) that supplies power to excite the superconductive coil 1, a perpetual current switch 3 connected in parallel with the superconductive coil 1 so as to create a perpetual current loop together with the superconductive coil 1, a heater 5 that heats a superconductive lead 4 incorporated within the perpetual current switch 3, a heater power source 6 that supplies power to the heater 5, a DC circuit breaker 7 that interrupts the power being supplied from the DC power source 2 to the superconductive coil 1 upon occurrence of abnormalities, and a protective resistor 8 that when the DC circuit breaker 7 is opened, consumes magnetic energy stored in the superconductive coil 1 so as to protect the superconductive coil 1. The superconductive coil 1 and the perpetual current switch 3 are incorporated within a cryogenic vessel (cryostat) 9 in order to realize a superconductive state. A shunt resistor 10 is used as a current detector to detect a current from the DC power source 2 in a constant current control state, and the thus detected power source current I.sub.1 is fed through an amplifier 11 into an adder 12.
The perpetual current switch 3 is constituted by the superconductive lead 4 and the heater 5, and functions such that when the heater power source 6 does not energize the heater 5, the superconductive lead 4 is refrigerated by liquid helium (LHe) (not shown) within the cryogenic vessel 9 to a temperature below the critical temperature Tc and becomes superconductive. When the heater power source 6 energizes the heater 5, the superconductive lead 4 is heated to a temperature above the critical temperature Tc so as to become normal-conductive, i.e., to possess a resistance value of R which is an electrically constant finite value.
FIG. 7 illustrates, with regard to the above-described configuration, the operation to create a perpetual current loop.
It is now assumed that both the superconductive coil 1 and the perpetual current switch 3 incorporated within the cryogenic vessel 9 have already been in a superconductive state and the DC power source 2 does not produce any voltage or current. In this state, when a current is fed into the superconductive coil 1, first, the heater power source 6 is turned on at a time T.sub.1, and this causes the heater 5 to heat the superconductive lead 4 within the perpetual current switch 3 so that the superconductive lead 4 becomes normal-conductive so as to possess the resistance value of R.
Next, at a time T.sub.2 the DC power source 2 is caused to start up and to produce voltage and current so as to energize the superconductive coil 1. At this instant, should a large current be abruptly supplied to the superconductive coil 1, a quenching is developed and the superconductive state thereof cannot be maintained, so that the DC power source 2 is controlled such that a current I.sub.3 to be fed into the superconductive coil 1 is gradually raised at a certain limited change rate. For this reason, a reference value I.sub.ref is fed into the adder 12 so that a power source current I.sub.1 of the DC power source 2 is changed from 0 to an ultimate target current value I.sub.O at a specified change rate. Namely, the DC power source 2 is operated in accordance with a difference .epsilon. between the reference value I.sub.ref and a detected current value (a feedback value) derived through an amplifier 11 from a shunt resistor 10 so as to control the power source current I.sub.1 which is fed into the superconductive current loop within the cryogenic vessel 9. This allows the power source current I.sub.1 to gradually increase from 0 to I.sub.O at a constant change rate according to the reference value I.sub.ref. During the period of this increase, the DC power source 2 generates a small constant voltage V, and in accordance with this voltage V, a small constant current I.sub.2 is fed into the superconductive lead 4. Therefore, into the superconductive coil 1, is fed the current I.sub.3 (I.sub.3 =I.sub.1 -I.sub.2) obtained by subtracting the small constant current I.sub.2 from the power source current I.sub.1 as shown in FIG. 7.
When the power source current I.sub.1 finally reaches, after continuously gradually increasing, the ultimate target current value I.sub.O, the output voltage V of the DC power source 2 becomes substantially zero because the difference .epsilon. becomes zero. This causes the current I.sub.2 which has been flowing through the superconductive lead 4 to decrease in accordance with a time constant (.tau.=L/R) determined by the resistance R of the superconductive lead 4 and the inductance L of the superconductive coil 1, and to become zero. Therefore, at this instant, the current I.sub.3 that flows through the superconductive coil 1 becomes equal to the power source current I.sub.1, i.e., to the ultimate target current value I.sub.O (I.sub.1 =I.sub.3 =I.sub.O).
Next, in this state, should the heater power source 6 be turned off at a time T.sub.3, the superconductive lead 4 is refrigerated by the liquid helium so as to become superconductive and to possess a resistance value of zero. After a time T.sub.4 at which the superconductive lead 4 possesses a resistance value of zero, the power source current I.sub.1 that flows into the DC power source 2 is gradually decreased. This decrease of the power source current I.sub.1 is fed into the superconductive lead 4 as a reverse-flow current I.sub.2. Namely, after the time T.sub.4, the superconductive coil current I.sub.3 does not change but flows separating into the power source current I.sub.1 and the superconductive lead current I.sub.2. As a result, a gradual decrease of the current I.sub.1 causes a gradual increase of the current I.sub.2 in a direction opposite to the arrow shown in FIG. 6. However, even in this case, there also exists a danger such that should the power source current I.sub.1 be abruptly decreased, the superconductive lead current I.sub.2 is caused to abruptly increase, whereby the superconductive lead 4 is returned to the normal-conductive state. Thus, there is performed a current control such that the power source current I.sub.1 is gradually lowered in accordance with the reference value I.sub.ref as shown in the dotted line in FIG. 7.
As a result, the power source current I.sub.1 becomes completely zero at a time T.sub.5, so that there can be obtained the superconducting perpetual current loop that circulates a perpetual current of I.sub.3 =I.sub.2 =I.sub.0 separated from the DC power source 2.
In a conventional medical diagnosis apparatus (NMR-CT) that employs a superconducting system, the nucleus to be imaged has been only one proton (i.e., the nucleus of hydrogen), and the apparatus has been generally of a type that is fixedly installed within a hospital's diagnosis room. There generally therefore has been no problem even when the strength of a generated magnetic field of the aforementioned superconductive coil 1 is maintained constant. Furthermore, the superconductive perpetual current loop generally has been maintained for a long period of time in which the diagnoses of a large number of patients has been successively made. In other words, frequent changes of the strength of the generated magnetic field of the superconducting system have generally not be necessitated.
However, in recent diagnostic apparata, besides a single proton, many other nuclei such as fluorine (F), sodium (Na) and phosphorus (P), for example, are required to be imaged so that more accurate diagnosis can be made. To realize this requirement, there should be created appropriate magnetic fields of strengths suitable for the nuclear magnetic resonance frequencies of the respective nuclei to be imaged. Naturally, the required magnetic field strength differs depending on the respective nuclei to be imaged, so that in order to obtain the imagery of the different nuclei, it is necessary to change the strength of the generated magnetic field of the superconducting system upon request.
Moreover, recently there is being studied a MRI diagnosis vehicle (nuclear magnetic resonance type medical diagnosis vehicle), which is large-size trailer on which the above-described medical diagnosis apparatus that includes the superconducting system is installed. This vehicle is mobile and goes to various places for the purpose of medical diagnosis. In this case, when moving from one place to another, the strength of the generated magnetic field of the superconducting system operated in the perpetual current loop state should be decreased to zero or extremely weakened. This is because the generated magnetic field for diagnosis has such a strength of 0.3-2.0 tesla (1 tesla=10,000 gauss) that the trailer cannot be relocated in view of the attendant danger to do so. Therefore, also in the case of MRI diagnosis vehicles it is necessary frequently to change the strength of the generated magnetic field of the superconducting system.
However, in the conventional method for controlling superconducting system, as shown in FIG. 7, the sweep gradient with which the DC power source 2 raises the current I.sub.3 of the superconductive coil 1 is disadvantageously identical to the sweep gradient with which the power source current I.sub.1 is lowered after the perpetual current loop is once obtained, so that a period from the time at which the specified perpetual current loop is obtained to the time at which the DC power source 2 is turned off becomes extremely long. When this disadvantage is considered in regard to the case of the aforementioned medical diagnosis apparatus (NMR-CT), there can be a case in which a series of the control time reaches several hours or even more. During this control time, naturally the diagnosis of the patient cannot be performed, and during this down time original diagnosis functions cannot be exhibited. This is a crucial disadvantage resulting in diagnostic inefficiency, and improvement thereof has been required.
Further, as shown in FIG. 7, during the period in which the current I.sub.3 of the superconductive coil 1 is raised by use of the DC power source 2, the heater power source 6 is maintained in the ON state so as to continue the state of heating the heater 5. This causes the inside of the cryogenic vessel 9 to be unnecessarily heated, so that expensive liquid helium, the refrigerant, is unnecessarily evaporated, disadvantageously limiting the useful life of the device in the system as well as having an adverse economical impact.